The pioneering water Cerenkov detectors, IMB [1]}, [2]} and Kamiokande [3]}, [4]}, observed a deficit in the up-going muon events generated by atmospheric neutrinos relative to down-going events.
It was hypothesized that the deficit was caused by neutrino oscillations.
This hypothesis was confirmed by the zenith angle dependence of event rates observed by Super Kamiokande [5]}.
Analysis of Super Kamiokande data gave
\(\Delta m^2_{\rm atm} \approx 3 \times 10^{-3}~ {\rm eV}^2 ~~~{\rm and} ~~~ \sin ^2 2\theta _{\rm atm} \approx 1,\) 
